Boron nitride (BN) is a synthetic material that has been prepared in different forms including hexagonal BN (hBN), rhombohedral BN (rBN), wurtzitic BN (wBN), and cubic BN (cBN). These allotropes are analogous to carbon structures and have sp2-hybridized (hBN and rBN) and sp3-hybridized (wBN and cBN) bonding phases. The most attractive BN allotrope is cBN which has properties similar to diamond as a consequence of high atomic density and strong covalent and ionic chemical bonds. Cubic BN has the second highest hardness and thermal conductivity next to diamond, but its chemical inertness to ferrous and steel materials at high temperature, resistance against oxidation and thermal stability make it superior to diamond and the best material in mechanical applications involving hard steels.
Coating machining tools with cBN films can improve their performance, but film thickness and film adherence are very important prerequisites in related mechanical applications. For instance, the coating thickness for cutting tools is typically 2-4 micrometers. However, cBN films are usually thinner and grown via soft and humidity-sensitive amorphous/turbostratic BN (aBN/tBN) precursor layers on many substrates. It is thus very challenging to prepare thick and adherent cBN films with long-term stability. In addition, energetic ion bombardment (tens to hundreds of eV) is still essential to facilitate cBN formation during the film growth, and consequently induces very high compressive stress, limiting the film thickness and contributing to film delamination. The compatibility of cBN and diamond, in terms of lattice parameter and physical properties, enables the heteroepitaxial growth of cBN films on diamond under proper conditions. Therefore, problems such as poor film adhesion and appearance of aBN/tBN interfacial layers have been solved by the introduction of polycrystalline diamond interfacial layers. However, the cBN growth process still cannot be accomplished without ion bombardment regardless of whether physical or plasma-enhanced chemical vapor deposition (PVD or PECVD) methods are used. The energetic particles promote the formation of sp3 bonding but they also induce considerable compressive stress. The high overall stress accumulates with increasing film thickness and may cause film delamination when the film thickness exceeds a certain critical value though cBN films can grow fairly thick on substrates coated with diamond.